This invention relates to a gas sensor comprising a semiconductor thin film that is heated to an operative temperature for detecting a species in an ambient gas. More particularly, this invention relates to a semiconductor gas sensor wherein the gas sensitive thin film is heated at a thermally isolated site to minimize the effects of heating upon the remainder of the sensor.
It is known to detect a species in a composite gas using a thin film of a suitable semiconductor material whose electrical resistivity changes in response to a presence of the species. For example, a semiconductor tin oxide thin film may be used to detect nitrogen oxides NO.sub.x ; see U.S. Pat. No. 4,169,369 issued to Chang in 1979. The electrical resistance of the film exposed to the gas is measured and provides a basis for determining the concentration of the species. The responsiveness of the film is temperature dependent. In general, it is necessary to heat the film to a predetermined elevated temperature to optimize the sensitivity for the particular species.
The delicate thin film is carried on a substrate having a substantially greater thickness to permit convenient handling. It is proposed to mount a plurality of semiconductor thin films on a common substrate. For example, a second film may be employed to detect interference due to a species other than the primary species of interest. The films, which may have like or unlike composition, may have distinct sensing temperatures. The substrate may also include an integrated circuit for interpreting the electrical resistance measurements, the operation of which may be adversely affected by heat. In order to better control temperature at other sites of the sensor, it is desired to limit the heating of the film to the immediate location of the film. However, heating the film necessarily results in heat loss to the underlying substrate, which tends to conduct heat to other locations of the device.
It is an object of this invention to provide an improved gas sensor device comprising at least one semiconductor thin film and means for heating the film to an operative temperature, which film and heating means are located at a site on a substrate that is thermally isolated from the remainder of the device to minimize heat conduction to other sites of the device. The substrate is etched to form a thin region at the site that reduces heat flow into the substrate and thus through the substrate into surrounding regions. Thus, the film is selectively heated, independent from elements located at other sites of the sensor.